Attack Chain

1. An unauthenticated attacker identifies a vulnerable FastGPT instance running a version prior to 4.14.9.5.
2. The attacker crafts a malicious HTTP POST request to the password-based login endpoint.
3. Within the POST request body, the attacker places a MongoDB query operator object (e.g., {"$ne": ""}) in the password field, bypassing the standard password check.
4. The vulnerable FastGPT application processes the malicious request without proper validation.
5. The MongoDB query operator is executed, bypassing the authentication mechanism.
6. The attacker is granted unauthorized access to the FastGPT application, assuming the identity of an arbitrary user, including the root administrator.
7. The attacker leverages their administrative privileges to access sensitive data, modify configurations, or perform other malicious actions within the FastGPT instance.

Impact

Successful exploitation of CVE-2026-40351 allows an unauthenticated attacker to gain complete control over a FastGPT instance. This can lead to unauthorized access to sensitive AI agent configurations, user data, and other critical information. The impact includes data breaches, service disruption, and potential compromise of downstream systems that rely on the FastGPT platform. Given the critical nature of AI agent building platforms, the compromise of a FastGPT instance can have far-reaching consequences.

Recommendation

• Immediately upgrade all FastGPT instances to version 4.14.9.5 or later to patch CVE-2026-40351.
• Deploy the Sigma rule Detect FastGPT NoSQL Injection Attempt to identify potential exploitation attempts targeting the login endpoint.
• Monitor web server logs for unusual POST requests to the login endpoint, specifically looking for MongoDB query operators within the password field as detected by rule Detect FastGPT NoSQL Injection Attempt.
• Review and restrict network access to the FastGPT instance to only authorized users and systems to minimize the attack surface.